The present invention relates generally to a sensor element that is primarily used for a thermal type flowmeter for measuring the flow rates of fluids, especially a gas such as air, and more specifically relates to such a sensor element that is excellent in functional properties such as a temperature coefficient of resistance.
A thermal type flowmeter is designed to measure the flow rate of a fluid by making use of the fact that the quantity of heat which is taken away from an electrically heated, heat-generating resistor by a surrounding fluid is variable due to the flow rate of that fluid. A sensor element used for this thermal type flowmeter is broken down into two types, one being the above-mentioned heat-generating resistor and the other a resistor for sensing the temperature of a fluid.
These sensor elements are required to have a large yet stable temperature coefficient of resistance; a resistance value as low as 5 to 30 .OMEGA. for the heat-generating resistor; and a resistance value as high as 100 to 1,000 .OMEGA. for the temperature-measuring resistance. These sensor elements are also required to have a low heat capacity as and respond quickly to the flow rate and temperature of a fluid.
A known example of the sensor element is an alumina pipe of 0.5 mm in diameter and 2 mm in length, with a platinum wire of 20 .mu.m in diameter wound there around. While this temperature coefficient of resistance is stable at a high value close to the bulk value of platinum due to the use of the platinum wire, this resistance value is low due to some limitation on the diameter of the platinum wire.
In order to overcome the problem mentioned above, it has been proposed to use a spirally wound thin film of platinum as a sensor element. However, a problem with the thin film of platinum is that its temperature coefficient of resistance is not only low but deviates largely as well, because it is likely to be affected by its production process and impurities.
A primary object of the invention, therefore, is to provide a sensor element that has a resistance value capable of being increased, as desired, and a large yet stable temperature coefficient of resistance and can be easily fabricated.